In drilling a well or other similar borehole, a drill bit is operatively coupled, usually by a drill string, to a drive which rotates the drill bit to cause the drill bit to bore into the earth. A drilling fluid, or drilling mud, is circulated through the borehole annulus. The drilling fluid passes through the drilling string and to the surface through the drill bit for cooling and lubricating the drill bit and for carrying rock cuttings generated by the cutting action of the bit to the surface. The drilling fluid may be a gas, but more typically is a liquid. Other types of drilling fluids are “drill-in” fluids, which are pumped through the drill pipe while drilling through the payzone, and “completion fluids,” which are pumped down a well after drilling has been completed.
Many liquids suitable for use as drilling fluids are known. Some drilling fluids are oil-based. Such oil-based fluids suffer from a number of drawbacks, particularly, their adverse effects on the environment. Oil-based fluids also can be costly to purchase and to dispose of.
Other drilling fluids are water-based or constitute an emulsion of oil in water or water in oil. Such fluids often are inexpensive compared to oil-based fluids, and are less costly to dispose of. Water-based fluids also pose less of a risk to the environment than do oil-based fluids. One drawback associated with water-based fluids is that the water in the fluid tends to promote borehole instability, particularly when shale is encountered in the drilling process. The water-based fluid may adsorb and absorb into pores in the shale, thus causing the shale to swell and thereby tending to cause the borehole to collapse.
One function of the drilling fluid is to provide hydrostatic pressure to counter the inward pressure exerted by the rock wellbore formation. Because the outward pressure of the drilling fluid generally is greater than the inward pressure of the rock formation, loss of the drilling fluid to the wellbore formation is common. As the fluid is lost to the wellbore, a filter cake composed of suspended and/or precipitated components of the drilling fluid typically builds up along the circumference of the borehole. This filter cake generally is undesirable. The filter cake cause “differential sticking” and other difficulties. In addition, the loss of fluid requires addition of more fluid, which can increase costs.
In recognition of the fluid loss problem, the prior art has provided numerous fluid loss control agents. Among such agents are lignites, chromium compounds, such as ferrochrome lignosulfonates, carboxymethyl cellulose, and starches, such as corn, potato, and tapioca starch. Starches used in prior art drilling fluids are sometimes derivatized, crosslinked, or otherwise modified to impart desired properties, such as viscosity and temperature stability.
The non-natural ingredients that heretofore have been used as fluid loss control agents are expensive and of potential environmental concern. The invention seeks to provide a natural fluid loss control agent that is comparable in properties to other known naturally derived fluid loss control agents.